Radical-clock reagents

On the other hand, participation of free radicals should be considered in most cases of oxygenations. Both of carbon and oxygen centered radicals can take part in the oxygenation processes. Apart from the apparent autoxidation process, it is not easy to differentiate explicitly metal-based mechanisms from free radical mechanisms. Different types of the radical-clock reagents have been developed for detection of radicals of different lifetime, especially in the discussions on the radical-rebound mechanisms. 

LFP-Clock Method. In this method, rate constants for the radical clock reactions are measured directly by LFP, and the clocks are used in conventional competition kinetic studies for the determination of second-order rate constants. The advantages are that the clock can be calibrated with good accuracy and precision in the solvent of interest, and light-absorbing reagents can be studied in the competition reactions. The method is especially useful when limited kinetic information is available for a class of radicals. 

The elucidation of mechanisms of reactions of Sml2 have involved polarography, kinetics, radical clocks and trapping techniques (radical cyclisation) [19, 20]. The reagent is able to reduce alkyl halides and ketones/aldehydes, as shown in Scheme 10.25, in non-chain radical reactions. 

N-F reagents acting as electrophiles, because radical clock experiments with these systems do not support a radical process [69, 70]. 

The term radical clock is used to describe a unimolecular radical reaction that is kinetically calibrated and, thus, can be applied in a competition study to time a particular radical reaction of interest [1], Such kinetic information is necessary for mechanistic studies where a radical might be formed as a transient. It is also important for synthetic applications because most radical-based methods involve chain reactions that commonly have several competing reaction steps with absolute kinetic values available, one can calculate the concentrations of reagents necessary for a high-yield synthetic conversion. Because lifetimes of simple radicals are usually in the microsecond range, direct kinetic measurements require sophisticated instrumentation. Radical clocks provide an inexpensive alternative for kinetic studies because the rate constants for the competing reactions are determined from the product mixtures present at the end of the reaction, usually with common organic laboratory instruments. 

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